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Charge transport optical characterization in semiconductor radiation detectors
Ridzoňová, Katarína ; Belas, Eduard (advisor) ; Toušek, Jiří (referee)
Measurements of DC photocurrent-voltage characteristic, photocurrents spectral response and laser induced transient current technique enable investigation of surface recombination centers, bulk trap levels and distribution of the inner electric field. In the presented work, the n- type planar CdZnTe detectors with quasi Ohmic contacts were studied by above mentioned methods. It has been shown that in the case of strongly absorbed light under the DC regime of illumination not only surface recombination influences the detector's transport properties. The effect of the space charge, created as a consequence of carriers trapped by the impurity levels, must be taken into account. Therefore some new theoretical models were created in order to describe measured photocurrent-voltage dependencies. Obtained data were fitted with the new theory and the mobility and surface recombination velocity for electrons were determined.
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Electrical and optical properties of ZnO single crystals
Zetek, Matyáš ; Belas, Eduard (advisor) ; Moravec, Pavel (referee)
In this thesis, we study electrical and optical properties of ZnO single crystals, by processing measurements of Hall effect to low temperatures. We also research how defects influence ZnO single crystal in terms of its electrical properties. We studied donors and acceptors levels in this material and their activation energies. We try to find what cause these donors and acceptors. We characterized material with its electrical conductivity, carrier concentration and carrier mobility. We carry an experiment of annealing ZnO in vapor of Zn, while we are looking for a change in mobility and a change in defect structure. As well we process photoluminescence measurement, the ZnO was excited with 355nm laser - light energy higher than the bandgap. We see green luminescence and its shift towards blue part of the spectra after annealing the crystals in ZnO vapor. Powered by TCPDF (www.tcpdf.org)
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Transport and optical properties of CdTe/CdZnTe single crystals
Uxa, Štěpán ; Belas, Eduard (advisor) ; Humlíček, Josef (referee) ; Drbohlav, Ondřej (referee)
Title: Transport and optical properties of CdTe/CdZnTe single crystals Author: Štěpán Uxa Department: Institute of Physics of Charles University Supervisor: Assoc. Prof. Eduard Belas, PhD, Institute of Physics of Charles University Abstract: The thesis is focused on a study of internal electric field and temperature dependence of the absorption edge of CdTe and CdZnTe samples. In the first part of the thesis the transient-current technique (TCT) is used for investigations of electric fields within planar radiation detectors. The original comprehensive theory that links together TCT measurements with measurements of detector's charge-collection efficiency (CCE) is presented. This approach results in the development of two new iterative methods for processing of experimental data which can be used in any situation when the internal electric field can be approximated by a linear profile. In the second part of the thesis high temperature measurements of transmittance of thin polished CdTe samples are presented, leading to the estimation of the temperature dependence of the bandgap energy from calculated spectra of absorption coefficient. This is for the first time when measurements in a Cd overpressure have been performed which significantly reduces sample sublimation. Keywords: CdTe, transient-current technique,...
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Gamma and X- ray CdTe/CdZnTe detectors
Pekárek, Jakub ; Belas, Eduard (advisor) ; Doležal, Zdeněk (referee)
In the present work we studied an influence of different types of surface etching and surface passivation of high resistivity CdZnTe-based semiconductor detector material. The aim was to find the optimal conditions to improve the properties of metal-semiconductor contact. The main effort was to reduce the leakage current and thus get better X-ray and gamma-ray spectrum, ie to create a detector operating at room temperature based on this semiconductor material with sufficient energy resolution and the maximum charge collection efficiency. Individual surface treatments were characterized by volt-ampere characteristics, spectral analysis and by determination of the profile of the internal electric field.
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Electromigration of defects in (CdZn)Te single crystals.
Pekárek, Jakub ; Belas, Eduard (advisor) ; Grill, Roman (referee)
In the present work we study effect of electromigration of defects in CdTe material in external electric field at various temperatures and biases. The aim was to verify this effect using a set of electric contacts arranged linearly along the sample. These contacts as a potential probes was used for the detection of the drift of the local resistance modulation. We try to determine under what conditions and why it happens and then decide how this effect can be used for purification of semi-conducting samples and for preparation of semi-insulating material.
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Dynamics of structural defects in CdTe-based semiconductors
Bugár, Marek ; Belas, Eduard (advisor) ; Holý, Václav (referee) ; Fochuk, Petro (referee)
Title: Dynamics of structural defects in CdTe-based semiconductors Author: RNDr. Marek Bugár Institute: Institute of Physics, Charles University in Prague Supervisor of the doctoral thesis: Doc. Ing. Eduard Belas CSc.; Institute of Physics, Charles University in Prague Abstract: The work was aimed at investigation of the effect of annealing on structural, electrical and optical properties of CdZnTe epitaxial substrates and CdTe-based and CdZnTe-based X-ray and gamma-ray detectors. The first part of the work is focused on investigation of structural properties of one type of second phase defects - inclusions - present in the material, which degrade the material quality. Consequent annealing experiments were aimed at reduction of these defects. In case of CdZnTe substrates, an annealing treatment leading to increase of the infrared transmittance was investigated. On the other hand, annealing experiments on the detectors of high-energetic radiation were focused on preservation of the high-resistive state. Moreover, the work contains detailed measurements of transport properties of CdTe taken directly at high temperatures. Key words: CdTe, annealing, inclusions, detectors, defects
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Diffusion of native defects and impurities in CdTe/CdZnTe
Šedivý, Lukáš ; Belas, Eduard (advisor) ; Šíma, Vladimír (referee)
Title: Diffusion of native defects and impurities in CdTe/CdZnTe. Author: Lukáš Šedivý Author's e-mail address: luky.sedivy@seznam.cz Department: Institute of Physics of Charles University Supervisor: Doc. Ing. Eduard Belas, CSc. Supervisor's e-mail address: belas@karlov.mff.cuni.cz Abstract: In this thesis, the influence of structural defects on the electrical and de- tection characteristics of CdTe material was investigated. The performed research fo- cused on the reduction of structural defects in the material by annealing in Cd or Te vapor, while preserving acceptable features for X-ray and γ-ray detection. The mate- rial was characterized by measurement of the electrical resistivity and concentration and mobility of free carriers. Tellurium and cadmium inclusions were studied using infrared microscope. The static and dynamic properties of defect structures at high temperatures and de- fined Cd pressures was investigated, as well, and chemical diffusion coeficients describing the dynamic properties of these defects were experimentally determined. Keywords: monocrystal CdTe, structural defects in semiconductors, annealing in Cd or Te, chemical diffusion coefficient, γ-ray detectors. 1
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Gamma- ray and X- ray detection by detectors based on (CdZn)Te single crystals.
Drobný, Miloslav ; Belas, Eduard (advisor) ; Grill, Roman (referee)
In this thesis, the influence of metal contacts, prepared on semiconductor detectors made of high resistive Cd0,85Zn0,15Te, on the quality of detected signal was investigated. The goal of thesis was to prepare ohmic contacts. Two materials (gold, platinum) and two methods of the preparation (chemical deposition, evaporation) were tested. It was found that improved detection properties were obtained on detectors prepared with gold contacts. Current-voltage characteristics of these contacts were found to be pseudoohmic. Moreover, when the Au contacts are prepared by evaporation, the value of the surface leakage current is lower.
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Influence of the annealing on the native defects concentration in (CdZn)Te semiconductors.
Bugár, Marek ; Belas, Eduard (advisor) ; Sopko, Bruno (referee)
Paper is focused on finding an optimal annealing treatment for elimination of concentration of inclusions in (CdZn)Te. Demands for material used for MBE growth of (HgCd)Te are material without crystal imperfections, infrared transmittance above 60% and good crystalline quality. These parameters were measured by the X-ray diffraction rocking curve (XRC) method, Infrared Fourier Transform spectrometer and Infrared microscopy. The influence of annealing in the range of 600-850řC in Cd overpressure was investigated on saples contained Te-rich inclusions. The change of shape of Te-rich inclusions and infrared transmittance after annealing in Tellurium vapor was measured. The influence of annealing in Te overpressure in the range of 600-800řC was also investigated o samples contained Cd-rich inclusions, one sample was annealed in Cd overpressure.
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